Optical communications systems presently contemplated use a light source and a photodetector that are optically coupled to each other by a glass transmission line which is commonly referred to as an optical fiber. Two types of devices, light emitting diodes and lasers, have received serious consideration as candidates for the light source. The laser is generally considered, especially at high data rates, to be the superior device.
Many semiconductor laser structures have been considered as candidates for the light source in such optical communications systems. At the present time, it is the consensus of those working in the art that buried heterostructure (BH) lasers are the leading candidates for optical communications systems because of their low current thresholds and stable operation up to high power. A BH laser is typically fabricated by growing a double heterostructure laser, etching a mesa, and cladding the sides of the mesa with semiconductor material grown during a second growth. The resulting laser is index guided with carriers confined to the active layer in both transverse directions. Such BH lasers have desirable characteristics such as a linear light versus current characteristic, stability against both kinks and pulsations in the absence of gross defects and no optical astigmatism.
An early article describing early work on BH lasers is Journal of Applied Physics, 45, pp. 4899-4906, Nov., 1974. This article describes an Al.sub.x Ga.sub.1-x As-GaAs BH laser which achieved lowest mode operation. Such operation is desirable because it insures that the light emission pattern, and thus the coupling efficiency between the laser and optical fiber, will not change. This operation was obtained using a very narrow and thin, desirably less than 0.4 .mu.m in both height and width, active region which was surrounded by material having a refractive index no more than 5 percent different from that of the active layer. This dimensional constraint makes it impractical to reliably construct the laser and also gives it a low power capability.
A later structure which is called a BH laser with buried optical guide (BOG) is described in Applied Physics Letters, 35, pp. 513-516, Oct. 1, 1979. This device uses a guiding layer adjacent to the active layer to widen the near field pattern perpendicular to the junction plane and thus increase the power that can be coupled into an optical film. The device can also use a stripe width as large as 4 .mu.m. Such a dimension is obtained only if the burying layer has a refractive index very close to that of the guiding layer.
A BH laser that currently represents the state of the art is the buried optical guide (BOG) laser, which is illustratively described in IEEE Journal of Quantum Electronics, QE-16, pp. 205-214, Feb., 1980. This laser incorporates a passive optical waveguide that results in the laser having a high differential quantum efficiency, low far field spreading and high catastrophic optical damage limits. The BOG laser has achieved thresholds as low as 10 mA/.mu.m of stripe width.
While perfectly adequate for many purposes, the BOG laser described suffers the drawback of involving critical fabrication steps. Although it is relatively simple to grow BH or BOG lasers with multimode waveguides because minimal constraints are imposed on the second or regrowth composition, it is difficult to make BH or BOG lasers with waveguides that are either single mode or with a gain of the fundamental transverse mode that is appreciably greater than any other mode without critical requirements being imposed on composition, dimensions, etc. In the BH and BOG lasers, the desired low mode operation is obtained by choosing the index of refraction of the second growth or outer cladding layer to be very slightly less than the effective index of the mesa layers. This imposes very critical limits on the composition of the regrowth layer. For example, for 2-3 .mu.m wide BOG lasers, the x of Al.sub.x Ga.sub.1-x As regrowth or second growth layer must be approximately only 1 to 2 percent more than the effective value of x, (x.sub.eff), for the mesa layers. Since x.sub.eff is a function of the dimensions and compositions of the layers of the central mesa, these layers must also be held to close tolerances. The percentage change in refractive index, n, is about 1/5 the percentage change in x, i.e., when n changes by 1 percent, x changes by 0.05 percent.